Many forms of naturally occurring cellulose are used to produce chemical pulps for the production of paper. The form used depends upon the availability of the material and the capability of the pulping equipment. One of the most common forms is the wood chip, either made from hardwoods or softwoods, but any other form of comminuted cellulose material may be used including grasses or agricultural waste, for example, bagasse and cornstalks.
An additional source of cellulose is the waste from saw mills, namely sawdust. Especially in lumber producing regions there is a plentiful supply of sawdust that can be pulped to produce wood pulp. The pulping of sawdust has both advantages and disadvantages. One advantage for using sawdust as a source of cellulose is that the smaller sawdust particles are relatively easy to impregnate with cooking liquor. For this reason the pretreatment systems for chemical pulping of sawdust are less complex than those used to impregnate wood chips, which are generally more difficult to impregnate than sawdust.
One disadvantage of chemical pulping sawdust is that sawdust can be resistant to the flow of cooking liquors. The finely dividing material tends to form a compact matrix when exposed to a liquid flow and limit flow through the material, if not prevent it altogether. For example, since batch digesters are highly dependent upon the capability of providing a cooking liquor circulation through the medium being pulped, it is difficult--if not impossible--to pulp sawdust in a conventional batch digester. Also, conventional continuous digesters, such as Kamyr.RTM. continuous digesters, also have difficulty handling sawdust without incorporating some form of special rotating liquid distribution device.
One common method used to continuously pulp sawdust is by using a drag-chain type digesters, for example, an M&D-type digester as shown in FIG. 138 of Volume 5 of TAPPI's Pulp and Paper Manufacture (1989), Grace, ed. These type of digesters consist of an inclined vessel through which sawdust is conveyed through the cooking liquor by means of a conveyor mechanism. However, this conveyor mechanism and its related hardware requires continuous maintenance that makes this type of system unsatisfactory in modern pulp mills.
Another mechanical disadvantage of the M&D-type digester for treating sawdust, and the like, is the rotary feed valve used. A typical device is shown in FIG. 139 of Grace. This rotary valve is a typical star-type feeder that inherently experiences an unbalanced pressure load due to the large pressure difference between the inlet and outlet of the valve. This load imbalance typically causes bearing wear requiring repeated maintenance.
In addition to the mechanical disadvantages, these M&D-type systems also have process disadvantages that make these systems less efficient than desired. One characteristic of the M&D-type process is the relatively short retention times. Two aspects of this type of digester limit the retention time: (a) steam heaving and (2) mechanical conveyance. Since the impermeability of sawdust prevents the sawdust from being heated by liquor displacement, the sawdust is heated by direct exposure to steam. The steam or vapor space required to expose the material to steam consumes some of the space that could be used for cooking retention time and hence limits the retention time.
The mechanical conveyor used in an M&D-type digester, referred to as a "drag conveyor", also limits the retention time because of the physical limitations of the size of the conveyor. It is simply too costly to manufacture a larger mechanical conveyor to achieve longer retention times.
As a result, the retention times provided by such a digester are limited to less than 1 hour, typically less than 30 minutes. Typically, additional cooking retention time is obtained when treating sawdust by following the M&D-type digester by ne or more retention vessels, or by "piggy-backing" two or more inclined digesters.
These characteristic short retention times also affect the cooking temperatures that are used. In order to obtain the proper degree of cooking, for example, to achieve a desired H factor, a relatively higher temperature must be used because of the shorter retention time. For example, if a typical cook requiring 2 hours retention time is limited to only 1/2 hour in an M&D-type digester, the cooking temperature must be increased from approximately 325.degree. F. to 360.degree. F. to achieve a comparable cook. this increase in cooking temperature increases the amount of high-pressure steam needed to maintain the higher cooking temperature. Therefore, the M&D-type digester is not as energy efficient as a digester capable of longer retention times.
These higher temperatures also consume more cooking chemicals and can potentially increase fiber damage. The rate of reaction of cooking chemicals with cellulose is highly dependent upon the prevailing temperature. The higher the temperature the faster and more aggressive the reaction. For kraft systems of the M&D type, the higher cooking temperatures, required for the shorter cooking times, result in higher reaction rates. This typically can cause increased chemical consumption and increased cellulose degradation.
The disadvantages of the M&D-type digester for cooking sawdust, and the like, are also seen in the "Pandia"-type digester shown in FIGS. 141 and 143 of Grace.
Another conventional continuous sawdust pulping system, shown in Grace, FIG. 133, and Smook, Handbook for Pulp and Paper Technologists, 1982, page 86 (FIGS. 8-17), comprises a cylindrical vessel fed by two horizontal screw conveyors and a pocket feeder, for example, a Kamyr.RTM. asthma feeder. This type of vessel is a steam-phase type in which a liquid level is maintained below the top of the vessel and steam is added to the space above the liquid level. The sawdust fed to this vessel by the pocket feeder is heated to cooking temperature by the added steam. This steam heating avoids the impractical practice of circulating heated liquor to heat to cooking temperature.
As described by Grace, the pulp in this type of digester is cooled by introducing wash filtrate to the bottom of the digester and extracting it by means of a centrally-located rotating cylindrical screen. (See U.S. Pat. No. 3,475,271 of Laakso.) However, due to the impermeability of finely divided material like sawdust, this method of extraction has been shown to be unstable.
This "asthma-feeder" style sawdust cooking system also has the disadvantage that the feed system is located above the digester vessel. This is because the asthma feeder is limited to transporting the sawdust a short distance. This limits the size and flexibility of such installations.
Another sawdust pulping system is shown in Canadian patent 1,242,055. This patent discloses the use of a conventional slurry pump to feed a slurry of sawdust and cooking liquor to a cylindrical digester. This transfer of medium consistency slurry by means of a pump prior to cooking is not energy efficient. Typically, such pumps are limited to medium consistency slurries of between 8 and 16% consistency. In heating such a slurry to cooking temperature the excess liquid volume must also be heated to cooking temperature. For example, a 12% slurry contains 7.33 lbs of liquid per lb. of fiber. In contrast, a 30% slurry contains 2.33 lbs. of liquid per lb. of fiber, or less than a third of the liquid per lb. of fiber. The lower consistency slurry requires additional energy to heat this excess liquid to cooking temperature.
Furthermore, no effort is made to minimize the mechanical action on the pulp or to recover heat from the cooked pulp slurry. Excessive mechanical action on sawdust slurries can be damaging to fiber properties, and is otherwise undesirable.
The present invention avoids these limitations of prior art continuous cooking systems for sawdust, and other finely divided comminuted fibrous material by first eliminating the need for high pressure mechanical feeders and conveyors; second, by discharging hot, pressurized cooked sawdust without cooling and without the aid of a rotating discharge device; and third by recovering the heat of the cooking reaction in an efficient economical manner.
The invention addresses the problems inherent in treating sawdust, or other finely divided source of cellulose material (which is within the scope of the term "sawdust" as used in the present specification and claims, e.g. initial cellulose particles which flow more like a powder than they flow like conventional wood chips), and provides for more efficient pulping, requiring less maintenance. The invention is practiced utilizing a static retention vessel. A "static" vessel is one without any significant internal circulation, which internal circulation typically include (in conventional continuous digesters for example) screens, conduits, pumps, heaters, and the like. While steam or heated liquid may be added to the pulp in the retention vessel, to ensure that it is retained at cooking temperature (although that is not normally necessary), there is no attempt to draw liquid uniformly through the vessel as in conventional batch and continuous digesters.
According to one aspect of the present invention a method of producing cellulose pulp from sawdust utilizing a static down-flow retention vessel is provided. The method comprises the steps of continuously: (a) Adding steam and cooking liquor to a flow of sawdust to produce a heated slurry of sawdust and cooking liquor at a consistency of between about 10-35%, preferably 20-30%, and a cooking temperature of between about 250-350.degree. F. (b) Passing the heated slurry from step (a) at superatmospheric pressure downwardly in the static down-flow retention vessel, and retaining the slurry in the retention vessel at cooking temperature between about 0.5-6 hours, and then discharging it at a consistency of between about 5-20% from the retention vessel. And, (c) at superatmospheric pressure, without significant (i.e. destructive to the fiber) reduction in pressure from the retention vessel, cooling the slurry discharged from the retention vessel by diffusing cooling liquid therethrough so that the temperature of the slurry drops below cooking temperature, and cooking thereof is terminated.
Step (b) is preferably practiced to discharge the slurry from the retention vessel without mechanically acting on the slurry (that is no mechanical agitator, pump, or like structure being provided). In fact it is desirable to discharge the slurry from the retention vessel substantially by gravity action alone (as by using a discharge having single convergence and side relief).
Step (a) may be practiced by initially forming a slurry at a first consistency greater than about 20%, and then successively: diluting and heating the slurry so that it has a readily pumpable second consistency of less than 20%; rethickening the slurry to a consistency of greater than about 20%; and then diluting and heating the slurry. Steps (a) through (c) are typically practiced to produce a chemical cellulose pulp having a Kappa No. of between about 10-30 (e.g. less than 24) with a yield of about 38-45% (e.g. about 39-42%).
There may also be the further step of pre-steaming the sawdust prior to step (a) in a steaming vessel and discharging the presteamed sawdust from the steaming vessel substantially by gravity action alone. There are also typically the further steps of washing and bleaching the pulp from step (c) depending upon the final product to be produced. Step (c) is also typically practiced by upflowing the suspension through a pressure diffuser at a consistency of about 5-20%. Step (a) is typically practiced to heat the slurry to a cooking temperature of between about 300-330.degree. F., and step (b) is practiced by maintaining the cooking temperature in the retention vessel about 1-3 hours.
Step (a) may be practiced by: diluting the slurry so that it has a diluted consistency of about 20% (e.g. about 10%) or less, and pumping the diluted consistency slurry to an elevated level near the top of or above the retention vessel; thickening the slurry at the elevated level to a consistency of about 20-40%; and steaming the thickened elevated slurry to increase the temperature thereof while diluting it to a consistency of about 5-20%.
According to another aspect of the present invention a system for (continuously) producing chemical pulp from sawdust is provided. The system preferably comprises the following components: A static down-flow superatmospheric pressure retention vessel having a top for receipt of a sawdust slurry, and a bottom for discharge of chemical pulp. A first mixer for mixing steam and cooking liquor with sawdust to form an initial slurry. Subsequent means for diluting, raising the temperature to cooking temperature, and pressurizing the initial slurry to provide a slurry suitable for cooking, and elevating the slurry to the top of the retention vessel to feed slurry into the top of the retention vessel. A non-mechanical discharge from the bottom of the retention vessel. And, a superatmospheric pressure vessel connected to the non-mechanical discharge for diffusing cooling liquid into pulp after the pulp is discharged from the bottom of the retention vessel to lower the temperature thereof below cooking temperature.
The subsequent means may comprise a thickener substantially at or above the top of the retention vessel, and connected to a steam mixer, the steam mixer connected to the top of the retention vessel and above it. The first mixer may comprise a screw conveyor mixer. The non-mechanical discharge may comprise a discharge with single-convergence and side relief. The subsequent means may comprise: a discharge chute having a top portion connected to the screw conveyor mixer, and a bottom portion; dilution liquid addition means to the discharge chute; a pump adjacent the discharge chute bottom portion and a conduit extending from the pump to the thickener; and/or dilution liquid addition means connected to the conduit from the pump. The superatmospheric pressure vessel preferably comprises a pressure diffuser.
The system may further comprise a second conduit from the thickener connected to the dilution liquid addition means to the conduit from the pump, and a heat exchanger for heating liquid in the second conduit disposed between the thickener and the dilution liquid addition means. A flash tank may be connected to the second conduit and includes a flash steam outlet and a liquid outlet, the flash steam outlet connected to the dilution liquid addition means to the discharge chute, and the flash steam outlet connected to the first mixer. Though the invention is disclosed for use with sawdust, one skilled in the art would recognize that for various aspects of the invention any other form of comminuted fibrous material may be used, for example, wood chips, agricultural waste or grass.
It is the primary object of the present invention to simply and effectively produce a relatively low Kappa No. chemical pulp, with relatively high yield, from sawdust. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.